Electronegativity.
Electronegativity can be defined as the measure of affinity of the element towards electrons . Higher the affinity towards electrons higher the electronegativity.
Eg : Fluorine has a high electronegativity. This is because it has 7 valence electrons, to gain a noble gas configuration / complete outer shell it needs to gain one electron. Hence the positive nuclear charge of the nucleus will attract an electron so it can become a complete outer shell , therefore F is very reactive.
Higher the attraction of the nucleus to electrons , higher its affinity therefore higher electronegativity.
